In such continuous molten steel casting of this type, various control methods have been heretofore proposed for optimally controlling the time of holding molten steel in a mold before the start of drawing out a dummy bar, for the purpose of attaining appropriate generation of a solidified shell. For example, Japanese Patent Unexamined Publication No. Sho-58-84652 has proposed a control method in which the quantity of molten steel to be injected and the target opening value of a sliding nozzle corresponding thereto are calculated from the depth of molten steel in a tundish on the basis of a predetermined ascending pattern of the molten-bath level in the mold to thereby control the quantity of molten steel to be injected. In this control method, however, the deviation of the molten-bath level with the passage of time from the predetermined molten-bath surface ascending pattern is not feedback-controlled. Accordingly, fluctuation due to the variation of nozzle characteristic and maloperation cannot be covered, so that there arises a mismatch state with respect to the flow rate.
Further, in order to improve the aforementioned control technique, Japanese Patent Unexamined Publication No. Sho-62-84862 has proposed a control method in which the time required for reaching a predetermined intermediate-check molten-bath level is set so that when the intermediate-check molten-bath level is not reached in the predetermined required time, this fact is used as a trigger to increase the opening of a flow rate controller up to a preset emergency processing opening to thereby follow a basic molten-bath level ascending pattern.
Further, Japanese Patent Unexamined Publication No. Sho-62-54562 has proposed a control method in which the molten-bath level ascending pattern is corrected when the molten-bath level ascending pattern is out of place at the intermediate-check level. Further, as the method of controlling the molten-bath level ascending rate, Japanese Patent Unexamined Publication Nos. Sho-62-183951, Hei-1-170568, Hei-2-142659, etc. have proposed various methods in any of which the detection level is grasped by the fact as to whether a predetermined molten-bath level is reached or not, so that feedback information of the detection level is not continuous.
Further, Japanese Patent Unexamined Publication No. Hei-2-142659 has proposed a control method in which a plurality of electrodes having different lengths are disposed so that respective molten-bath levels are detected. In this control method, however, the following disadvantages are pointed out.
(1) The cost of capital investment becomes high. PA0 (2) The influence of the maloperation due to the influence of a splash cannot be removed completely. PA0 (3) Running cost becomes high. PA0 (4) In a billet continuous casting process, it is difficult, from limitation of equipment, to mount a plurality of electrodes in a small sectional area, for example, having a diameter not larger than 170 mm.
As described above, a system in which the ascending rate of the molten-bath level is measured every moment continuously from the time point just after the start of injection to thereby perform feedback control, is not employed in the conventional control methods. There is however some inclusion in molten steel in a tundish, so that the inclusion near the upper portion of the molten steel in the tundish is caught in if the molten-bath level ascending rate just after the start of injection of molten metal is too high. This causes defectives such as cracking of the billet due to the inclusion after casting. There arises a problem that defective percentage becomes high if the molten-bath level ascending rate is set to the optimum value. This problem is remarkable particularly in the case of billet continuous casting by which the molten-bath level ascending rate in the mold with a small section just after the start of injection of molten steel into the mold is high.
Further, in the case where the tundish is re-used as in the case of slab continuous casting, the nozzle gain is changed widely by the influence of slag remaining in the tundish just after the start of casting, so that the discharge flow rate is changed. Accordingly, automatic starting cannot be performed stably without feedback control in the control region. Therefore, the sliding nozzle cannot but be operated manually. In the case of such a manual operation, there is however a tendency of overaction, so that the frequency in generation of the trouble of choking of the nozzle is high.